1. Field of the Invention
This invention generally relates to a signal transmission structure, and more particularly to a signal transmission structure for improving the mismatch of the characteristic impedance of the signal transmission structure when the signal trace passes through a non-reference region.
2. Description of Related Art
On a printed circuit board and packaging substrate, the signal trace used for connecting two devices or two terminals has to maintain a uniform trace width in order to keep a constant characteristic impedance when the electronic signals are transmitted on the signal trace. Especially when the signals are transmitted in a high speed and a high frequency environment, a good impedance matching design between the two terminals are required to reduce the reflection due to the impedance mismatch, i.e., to reduce the insertion loss and increase the return loss when transmitting the signals so that the quality of the signal transmission will not be affected.
FIGS. 1A and 1B show a top view and a side view of a conventional signal trace through a non-reference region. The signal transmission structure 110 at least includes a reference plane 120 and two signal traces 130 and 140. The reference plane 120 for example is a power plane or a ground plane. The signal traces 130 and 140 have a uniform trace width. It should be noted that in the conventional circuit design, the reference plane will have a plurality of through holes due to the hole drilling or cutting between the planes, or will have a non-reference region 122 (such as a non-reference region opening). Hence, when the signals are transmitted on the signal traces 130 and 140, a high impedance will occurs at the non-reference region 122. Further, the coupling inductance between the signal traces 130 and 140 also increases so that the effect of the near-end crosstalk and the far-end crosstalk becomes more serious. Therefore, the signals cannot be completely transmitted from the one terminal of the signal traces 130 and 140 to the other terminal.
FIG. 2 shows the relationship between the characteristic impedance and the frequency when the conventional signal trace passes through a reference plane (solid line R) and a non-reference region (solid line T), respectively. Referring to FIGS. 1A and 2, when the working frequency is higher, the characteristic impedance is higher when the conventional signal traces 130 and 140 at the same frequency pass through a non-reference region 122. Hence, the impedance mismatch occurs. Therefore, when the signal traces pass through an incomplete reference plane 120, the characteristic impedance of the signal traces 130 and 140 increase as the frequency increases. Hence, the difference between the original characteristic impedance and the affected characteristic impedance also increases, which causes a more serious characteristic impedance mismatch on the signal traces 130 and 140.